Article detection and/or recognition using magnetic devices

ABSTRACT

This application discloses a method of recognizing and/or locating preselected categories of articles, which comprises applying to the articles a plurality of magnetic elements in predetermined associations (e.g. with predetermined numbers of magnetic elements and with predetermined spacings between said elements), whereby when the articles are caused to move relative to a predetermined interrogating magnetic field, each particular association of magnetic elements gives rise to a magnetic signature whereby the article or category of article carrying each of the predetermined associations can be recognized and/or located. A series of tags or markers for use in such a method, as well as a system for determining the location and/or identity of an article within an article conveying network, are also disclosed. Typically, one of such a series of tags will have a plurality of magnetic elements (2, 3, 4, 5, 6, 7, 8, 9, 10) whose individual magnetic characteristics, and whose number, shape, location and orientation on a substrate, determine the individual character of the magnetic response of the tag. By varying one or more of these parameters, a large family of magnetically individually recognizable tags can be generated.

This invention relates to magnetic devices and to their use in articlerecognition and/or detection.

The use of magnetic devices to act as antipilferage tags is well known.Typically, the tag is a magnetic medium which is detected when it (andthe article to which it is attached) passes through a detection system,typically in the form of a walk-through framework which emits analternating magnetic interrogation field. This field is designed tointeract with a tag which has not been subjected to the routinedeactivation procedure and to respond by, for example, triggerring awarning signal in the event that detection of a non-deactivated tagoccurs. The tag can be deactivated when a shop assistant carries out theroutine procedure at the time of effecting a sale. Deactivation isusually effected by applying a magnetic field to the tag which itselfincludes a deactivation layer, generally in the form of a magneticallysemi-hard material with a high coercive force, located close to theactive element in the tag. The semi-hard deactivating layer can bemagnetised by a strong magnetic field and as a result the magnetiseddeactivating layer prevents the magnetically soft active layer fromresponding when subjected to an alternating magnetic field. Suchdeactivation prevents detection of the magnetic tag by the detectionsystem.

The shape of the material making up the active element of the magneticmarker strongly affects the magnetization response to an externalmagnetic field, because of the demagnetization factor N, which isdependent on the shape. Known markers take the form of amorphous metalribbon ferromagnets which are formed by melt-spinning or similartechniques. These ribbons are relatively thick, generally over 10microns and often about 25 microns in thickness.

The shape-dependent demagnetizing field ΔH is equal to the product ofthe shape-dependent demagnetization factor N and the intensity ofmagnetization M.

The effective permeability (μ_(e)) of the tag can be derivedapproximately by the following formula: ##EQU1## where μ_(i) is theintrinsic permeabilty of the magnetic coating, and N is thedemagnetisation factor; this (N) can be calculated as a function of theshape of the article. The inverse of the demagnetisation factor can betermed the shape factor (1/N).

The effective permeability of the active component of a tag thus dependsnot only on the intrinsic permeability of the material of which it isformed, but also on its shape. The lower the demagnetisation factor, thecloser is the effective permeability to the intrinsic permeability. Lowdemagnetisation factors are also desirable since they permit a lowerintensity interrogation field to be used.

We have discovered that the use of similar magnetic devices can beextended to detection/recognition systems, by means of which differenttypes of magnetic device can be distinguished and hence also the articleor class of article on which they are carried may be distinguished andidentified, or the location of a given type of article may be found.

In particular, several elements with either different ferromagneticmaterial compositions and hence a variety of magnetically nonlinearcharacteristics, or several different magnetically soft and/or hardand/or semi-hard elements in proximity to one another can be used. Thehard or semi-hard magnetic material acts (to a greater or lesser extentdepending on magnetic and physical parameters such as coercivity, volumeand spacing) as a clamp which holds the soft material in a fixedmagnetic state in which there is a specific response to an interrogating(generally alternating) field. Various combinations of the differentelements thus characterise different objects, and give rise to specificmagnetic signatures.

According to one aspect of the present invention, there is provided amethod of recognising and/or locating preselected categories ofarticles, which comprises applying to the articles a plurality ofmagnetic elements in predetermined associations (e.g. with predeterminednumbers of magnetic elements and with predetermined spacings betweensaid elements), whereby when the articles are caused to interact with apredetermined interrogating magnetic field, each particular associationof magnetic elements gives rise to a magnetic signature whereby thearticle or category of article carrying each of the predeterminedassociations can be recognised and/or located.

Examples of such associations include (1) sets of hard and soft magneticelement pairs where the hard elements have various remanentmagnetizations, and (2) several soft magnetic elements spaced variousdistances from one or more hard magnetic elements. The hard (permanent)magnetic element serves to bias the magnetic response of a soft elementsuch that the centre of the hysteresis loop is displaced from zeroapplied field, so that a small constant or quasi-static external field,H_(x) must be applied to excite the full non-linear properties of thesoft element. In associations of type (1), the various pairs, with softelements all spaced identically from their respective hard element, willshow various H_(x) because of the various magnetizations shown by thehard elements, different combinations of which can characterisedifferent objects. In associations of type (2), the various softelements will experience different bias fields and show a variety ofH_(x) because of the difference in the distances to the biasing element.

The or each soft magnetic element may be an amorphous metal ribbon,typically from 10 to 50 microns in thickness, and which may be formed,for example, by melt spinning. Alternatively, the or each soft magneticelement may be a thin coating of an amorphous metal glass of highintrinsic magnetic permeability, with low or substantially zeromagnetostriction, and with low coercivity; and which is preferably notgreater than 6 microns in thickness.

According to a second aspect of the present invention there is provideda method of controlling the nonlinear magnetic characteristics of amagnetic marker or tag, which comprises applying to an article aplurality of magnetic elements of selected shapes and/or withpredetermined magnetic characteristics and/or spacings.

Hence, it is proposed to control nonlinear magnetic characteristics bycontrol inter alia of the element shape and hence the demagnetizationfactor N. Thus a variety of elements of different shape (hence ofdifferent N) can be used for article coding. Preferably the markers areapproximately ellipsoidally shaped elements, and/or are of a shape inwhich the demagnetization factors along two or more mutually angled(e.g. mutually orthogonal) directions are preselected. This could alsoinvolve the use of thin film and/or thick magnetic markers.

The nonlinear characteristics of soft ferromagnetic tagging elements areusually detected and measured by a rapidly varying magnetic field of oneor more frequencies f_(d) in the range 500 Hz to 100 kHz applied over alarge interrogation volume. Because the soft magnetic element has anonlinear magnetic response, signal components which are not present inthe interrogation field, are generated. These signals, which areharmonically related to the frequency or frequencies f_(d) are detectedto indicate the presence of the soft magnetic element. The specialindentification configurations described above are characterised by adisplacement (due to H_(x)) or a shear (due to N) of the hysteresis loopof the soft magnetic material. The article recognition system needs tobe capable of distinguishing these changes in the nonlinear magneticcharacteristics of the soft element. This can be achieved by theaddition to the interrogating field of a slowly scanned bias field suchas a slowly varying magnetic field with a frequency f_(s) below 500 Hzand advantageously below 100 Hz. The value of the bias field at whichthe nonlinear response of each soft element is maximized can be measuredand used to characterise each element. The phase and amplitude of thedetected signal frequencies, f_(d), as a function of the amplitude ofthe scanning field at the low frequency, f_(s), can be used as a moresophisticated characterising feature.

According to another aspect of the present invention, there is provideda method of recognising and/or detecting an article at a given point inthe course of its movement from a first location to a second location,which comprises securing to said article while it is at said firstlocation a magnetic device which comprises a plurality of magneticelements in a predetermined association (e.g. with predetermined numbersof magnetic elements and with predetermined spacings between saidelements), at least one of said magnetic elements being a soft magneticmaterial with low coercivity, high intrinsic permeability and low orzero magnetostriction, and at least one other of said magnetic elementsbeing a semi-hard magnetic material; subjecting the article carryingsaid magnetic device to a predetermined interrogating magnetic field atsaid given point and observing the interaction between the interrogatingfield and the magnetic device carried by the article; deducing from saidobserved interaction the nature of the article; and allowing the articleto travel from said given point to said second location.

In one embodiment of the invention, the first location is a store wherea multiplicity of articles of various types are held for distribution;for example, the store of articles may be a warehouse or the stock roomof a retail shop. Each different type of article (identified accordingto any convenient system -- e.g. by the character of the goods; by theirprice, origin, date of manufacture and/or packing, `use by` date, andcontents; or by combinations of such parameters) will carry a magneticdevice characteristic of its type. Different characteristics areachieved by adopting different associations between magnetic elements.Thus by using a predetermined number of magnetic elements and apredetermined number of mutual orientations/spacings between respectiveelements in the magnetic device, a large number of permutations ispossible each of which has a specific magnetic `signature` -- i.e. aspecific, detectable interaction with the interrogating field which canbe differentiated from the interaction of the other magnetic deviceswith the same interrogating field. With such a system, the `given point`may be a point of sale within a shop, and the `second location` may bethe exit from the shop.

In an alternative embodiment, the first location may be a distributionpoint for letters, packages and the like; the given point may be asorting office; and the second location may be the address to which thearticles are to be despatched.

In a third embodiment, the first point can be a point of access to anunderground pipe system, the given point may be at a site where thecontents of the pipe system are inaccessible, but the pipe itself isaccessible; and the second location can be any point downstream of thegiven point. With this embodiment, the passage of a given item orarticle through the pipe system may be monitored at the given locationby magnetic means.

It will be appreciated from the above embodiments, which are given byway of example only, that the method of the invention is susceptible ofapplication in diverse fields of commercial and technical significance.

According to a fourth aspect of the present invention, there is provideda series of magnetic tags or markers which are magneticallydistinguishable from each other, characterised in that each tag ormarker carries a plurality of magnetic elements including at least onesoft magnetic element, the plurality of elements in each of theindividually distinguishable tags or markers having a characteristicmagnetic signature when subjected to an alternating magnetic field byvirtue of at least one of (a) their characteristic magnetic properties;(b) their location; (c) their shape; and (d) their orientation withrespect to one another on the tag or marker. A series of such tags ormarkers will generally be such that each of the magneticallydistinguishable tags or markers includes at least one soft magneticelement and at least one hard or semi-hard magnetic element.

According to a fifth aspect of the present invention, there is provideda system for determining the location and/or identity of an articlewithin an article conveying network, which comprises a series of tags asdefined above, and a detection system for detection and measurement ofthe magnetic characteristics of said tags or markers when subjected toan alternating magnetic field.

According to a sixth aspect of the present invention there is provided amagnetic tag or marker comprising a magnetic circuit of magnetostrictivematerial and having a discontinuity the value of which varies underexcitation so as to alter the magnetic field of the magnetic marker.

Such a marker operates on the principle of magnetic circuit modulation.Mechanical resonances of elements with magnetostrictive characteristicshave been used previously, with detection of sound waves emitted. Incontrast the embodiments of this aspect of the invention are directed tothe use of such mechanical resonances (excited either by acoustic ormagnetic fields) to produce a corresponding resonance in magneticproperties, which may be detected by magnetic or electromagnetic means.This is achieved in an embodiment by a marker in the form of a magneticcircuit, such as an annulus, with a small break in circuit continuity,such as a fine cut or gap. Mechanical oscillation of this gap will leadto a change in the magnetic field of the magnetic circuit of the marker,which may be detected remotely. This detected signal will be strongestat a frequency related to the mechanical resonance frequency, so thatseveral such elements of different resonance characteristics, inconjunction with a swept or multifrequency interrogation field, can beused for article coding or recognition.

In addition, deactivating elements may be used which are magneticallyselectable either by using materials of different coercive force, orelements with different demagnetization factors This would enable theelements characterised by different frequencies to be selectivelydeactivated, so that article recognition codes may be changed by theuser. Such selective deactivation principles are also applicable todeactivatable markers described above and to those known from the priorart.

For a better understanding of the invention, and to show how the samemay be put into effect, reference will now be made, by way of example,to the accompanying drawings, in which:

FIG. 1 is a schematic hysteresis plot (intensity cf magnetisation, M,plotted against applied field, H) illustrating one mechanism which maybe utilised in the method of this invention;

FIG. 2 is a second schematic hysteresis plot showing a second mechanismwhich may be utilised in the present invention; and

FIGS. 3-5 illustrate different embodiments of tags or markers which maybe utilised in the method of this invention.

Referring now to FIG. 1, the effect of a hard magnetic element acting ona soft magnetic element is shown. As explained hereinbefore, the hard(permanent) magnetic element serves to bias the magnetic response of thesoft magnetic element so that the centre of the hysteresis loop isdisplaced from zero applied field (H) by a value H_(x).

Referring now to FIG. 2, two hysteresis plots are shown; the steeperplot Pl represents a typical plot for a soft magnetic material with alow demagnetisation factor, N; and the more oblique plot P₂ represents atypical plot for a soft magnetic material with a higher value of N. Asexplained hereinabove, the degmagnetisation factor, N, isshape-dependent and consequently the hysteresis plot in any given casecan be altered in a highly specific manner by altering the shape and/ororientation of one or more soft magnetic elements located in relativelyclose proximity. This mechanism, together with the magnetic bias effectillustrated in FIG. 1, make it possible to produce a very large numberof tags or markers which are recognisably distinct by virtue of theirmagnetic properties, but which can be produced relatively simply.

In FIGS. 3-5, three different configurations of magnetic elements (fromthe almost limitless variety which can be produced) are shown. In eachof these FIGURES, a substrate 1 carries a series of magnetic strips orzones 2-6 together with (in FIGS. 1 and 2) one or more hard magneticstrips or zones 7-10. The following Examples illustrate the fabricationand functioning of these particular tags:

EXAMPLE 1

This Example describes the production of a coded tag as shown in FIG. 3consisting of three components: two soft ferromagnetic elements (2; 3)and a hard magnetic biassing element (7).

The soft magnetic elements 2 and 3 were fabricated from strips of a meltspun amorphous alloy produced by Vacuumschmeltze, designated Vitrovac6025. The strips were 0.8 mm wide and 25 microns thick. The hardbiassing element 7 was a strip of ductile permanent magnetic material,Crovac 250, also produced by Vacuumschmeltze. The permanent magnet strip7 was 1 mm wide and 50 microns thick. All magnetic elements wereinitially in the form of reels of the strip material, about 500 m long.The strip of Crovac was fully magnetized along its length by a largecurrent pulse passing through a copper loop which linked the reel ofstrip. The three elements were then laminated onto a 2 cm wide polymercarrier sheet 1 consisting of 100 micron thick "Mylar" coated with athin layer of thermoplastic glue. The soft magnetic elements 2 and 3were laminated onto the sheet so as to lie parallel to the permanentmagnet strip, with one soft strip 2 placed at a distance of 0.8 mm fromthe adjacent edge of the permanent magnet strip 7, and the second strip3 at 10 mm from strip 7. This spacing was achieved by the use of narrowguide pulleys, close to the point of lamination across which the stripswere led as they were being unwound from the reels. A 2 cm wide facepaper sheet (not shown) was laminated onto the assembly immediatelyafter the magnetic strips, to disguise and protect the magnetic elements2, 3 and 7. The completed strip was finally cut into 5 cm lengths whichare used as the identifier labels.

The effect of this arrangement was to produce a displacement H_(x) inthe hysteresis loop (see FIG. 1) of approximately 100 A/m in the softstrip closest to the permanent magnet, and only 10 A/m in the moredistant strip. Variations in the separations of the strips, achieved byadjustment to the positioning pulleys, led to tags with differentcombinations of H_(x) and hence different codings.

EXAMPLE 2

The procedure of Example 1 was repeated but pairs of 5 cm of themagnetic elements lengths were also assembled orthogonal to each otherin the form of a cross. This assembly reduces spatial orientationeffects in the interrogation field.

EXAMPLE 3

The procedure of Example 1 was repeated except that three soft magneticstrips 2, 3 and 4 each 0.8 mm wide and spaced apart by 8 mm were securedto the substrate 1. Three hard magnetic elements 8, 9 and 10 werelikewise deposited on substrate 1. Each of these was 2 mm wide, and thegap between adjacent edges of strips 2 and 8, 3 and 9 and 4 and 10,respectively was 0.8 mm. Strip 8 was 1 cm long; strip 9 was 2 cm long;and strip 10 was 4 cm long. Variations in strip dimensions andseparations allow a very large number of uniquely identifiable tags tobe produced. EXAMPLE 4

This Example describes the production of a thin film tag consisting ofseveral thin film soft magnetic components of various shapes. The softmagnetic components were made by sputtering a layer of amorphous metalglass onto a polymer film and cutting the desired shapes from the coatedfilm.

Sputtering is a physical vapour deposition process, carried out in avacuum chamber, in which ions of gas, usually argon, are acceleratedacross a potential difference with sufficient force to eject atoms froma target. The ejected atoms travel through the partial vacuum until theycollide with a surface on which they can condense forming a coating. Inthis example, the target was an alloy capable of forming an amorphousmetal glass and consisting of cobalt (66 atom %), iron (4 atommolybdenum (2 atom %), silicon (16 atom %) and boron (12 atom %). Thetargets were manufactured by hot isostatic pressing (HIPing). With thistechnique, an initial ingot was made from the elements by inductionmelting under vacuum. The ingot was pulverised to yield a powder withmaximum particle size 0.5 mm diameter. The powder was then thoroughlymixed to ensure even distribution of elements. Next, stainless steel`cans` were manufactured to contain the powder during the HIPingprocess. The cans were formed from a steel block containing a trough tohold the powder, dimensions being determined by the planar magnetronused for sputtering, the trough being the width of the magnetron's `racetrack`. The ends of the trough were closed off with stainless steelblocks and it was then filled with the powdered alloy. The surface wascovered with a thin stainless steel sheet and sealed under a vacuum of5× 10⁻⁴ torr using electron beam welding.

The HIP process was carried out at 1000° C.±10° C. and 103 MPa for 120minutes; this resulted in 100% consolidation of the powder to acrystalline block with uniform distribution of the elements.

Finally the cans were machined to expose the consolidated powder. Thestainless steel surround was then shaped to form a backing plate for thetarget and also a side support for clamping to the magnetron. A numberof target tiles manufactured in this way can be butted together to fitany size of magnetron.

The soft magnetic layer was manufactured by sputtering a layer of theamorphous alloy one micron thick onto a continuous web of the polymer`Upilex` (a cast polyimide manufactured by ICI and resistant to heat upto a temperature of 250° C.). Sputtering took place using a low basepressure of around 10⁻⁶ torr and a purified argon gas supply. A lowsputtering pressure of around 0.5 Pascals yielded a compact film which,in the finished article, gave a high magnetic permeability.

Elliptical shapes were then cut from the soft magnetic film withappropriately shaped die cutters. Ellipses used were 8 mm major axis and4 mm minor axis. Sets of two ellipses (5; 6) were then laminated onto acarrier sheet 1 with glue (see FIG. 5) similar to that described inExample 1, though 4 cm wide. Similarly, the magnetic elements wereprotected by application of a 4 cm wide face sheet (not shown). The twoellipses 5 and 6 were separated on the lamination by 2 cm, andpositioned so that their major axes were mutually orthogonal. Thelaminated sheet was then cut into 4 cm lengths, each containing one setof two ellipses 5 and 6 as shown in FIG. 5.

The finished marker was such that the two soft elements presented twodistinctly different hysteresis loops along either of two orthogonaldirections. Alterations to the coding were achieved by using alternativeelliptical shapes such as 12 mm major axis and 3 mm minor axis.

We claim:
 1. A method of recognising and/or detecting an article at agiven point in the course of its movement from a first location to asecond location, which comprises securing to said article while it is atsaid first location a magnetic device which comprises a plurality ofmagnetic elements in a predetermined association (e.g. with predeterminenumbers of magnetic elements and with predetermined spacings betweensaid elements), at least one of said magnetic elements being a softmagnetic material with low coercivity, high intrinsic permeability andlow or zero magnetostriction, and at least one other of said magneticelements being a semi-hard magnetic material; subjecting the articlecarrying said magnetic device to a predetermined interrogating magneticfield at said given point and observing the interaction between theinterrogating field and the magnetic device carried by the article;deducing from said observed interaction the nature of the article; andallowing the article to travel from said given point to said secondlocation; said first location being a point of access to an undergroundpipe system and said given point being a site where said pipe systemitself is accessible but where the contents of said pipe system areinaccessible.
 2. A method as defined in claim 1 wherein tags or markersare attached to, or associated with, a plurality of said articles,characterised in that each tag or marker carries a plurality of magneticelements including at least one soft magnetic element, the plurality ofelements in each of the individually distinguishable tags or markershaving a characteristic magnetic signature when subjected to analternating magnetic field by virtue of at least one of (a) theircharacteristic magnetic properties; (b) their location with respect toone another on the tag or marker; (c) their shape; and (d) theirorientation with respect to one another on the tag or marker.
 3. Amethod as defined in claim 1 wherein tags or markers are attached to orassociated with a plurality of said articles, comprising an element ofmagnetostrictive material and having a discontinuity the value of whichvaries under excitation so as to alter the magnetic field of themagnetic marker, the magnetostrictive element being fabricated so thatit has a mechanical response the frequency of which also characterisesthe magnetic response of, and which is a recognisable characteristic of,said element.
 4. A method of recognising and/or locating and/ordetecting an article in an underground pipe system, which comprises:(1)positioning said article at a known, accessible first position; (2)fixing to said article while it is at said first position a magneticdevice which comprises a plurality of magnetic elements in apredetermined association (e.g. with predetermined numbers of magneticelements and with predetermined spacings between said elements), atleast one of the magnetic elements being a soft magnetic material withlow coercivity, high intrinsic permeability and low or zeromagnetostriction, and at least one other of said elements being asemi-hard magnetic material; (3) subjecting the article carrying saidmagnetic device to a predetermined interrogating magnetic field while itis at said first position and observing the interaction between theinterrogating field and the magnetic device carried by said article todetermine the magnetic signature of the combination of said article andsaid magnetic device; and (4) after said article has been introducedinto said underground pipe system, and when it is desired to recogniseand/or locate an/or detect said article within the underground pipesystem, subjecting a portion of said underground pipe system to saidpredetermined interrogating field in order to detect the presence orabsence of said magnetic signature.
 5. A method as defined in claim 4wherein tags or markers are attached to, or associated with, a pluralityof said articles, characterised in that each tag or marker carries aplurality of magnetic elements including at least one soft magneticelement, the plurality of elements in each of the individuallydistinguishable tags or markers having a characteristic magneticsignature when subjected to an alternating magnetic field by virtue ofat least one of (a) their characteristic magnetic properties; (b) theirlocation with respect to one another on the tag or marker; (c) theirshape; and (d) their orientation with respect to one another on the tagor marker.
 6. A method as defined in claim 4 wherein tags or markers areattached to or associated with a plurality of said articles, comprisingan element of magnetostrictive material and having a discontinuity, thevalue of which varies under excitation, so as to alter the magneticfield of the magnetic marker, the magnetostrictive element beingfabricated so that it has a mechanical response, the frequency of whichalso characterises the magnetic response of, and which is a recognisablecharacteristic of, said element.